Importance

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Abstract

In equipment which comprises a storage station or portable data carrier holding data and a control station for interaction with the storage station to alter the stored data, for example telephone equipment in which the user carries a pay token for use in pay phone equipment which changes the value recorded in the token in accordance with the call made, a security check operates by means of a reference signal and a test signal held in the storage station, the reference signal selecting a corresponding test signal in the control station for comparison with that of the storage station. The control station in one mode will present a valid test signal for comparison and in another mode will present an invalid test signal. These modes may occur in different sequences and the control station evaluates the responses of the storage station to both valid and invalid test signals before allowing use of the data held by the storage station. After each transaction the reference signal and test signal held by the storage station are replaced by new signals generated by the control station. The test signals held by the control station are periodically changed so that some are available only for checking and not for re-use while others are no longer available and a storage station holding an out-of-date test signal therefore cannot be used.

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